The invention relates to wells and particularly to the rebuilding or evaluating of existing water wells. Many existing water wells are drilled in water-bearing, permeable rock such as sandstone or limestone. Presumably, the rock apparently seemed quite stable when these wells were drilled since a large number of them were completed without having well screens affixed at the lower ends of their steel casings. Such "open hole" wells, after years of use, often tend to pump sand as the continued flow of water through the rock develops fissures and fractures of the rock. Since such a condition cannot be tolerated, it is desirable to rehabilitate the well by installing a screen in the open hole beneath the casing and a graded gravel pack in the annulus between the screen and the sides of the hole. The screen would typically have a diameter about 2" less than the casing and would have a riser attached at its top which would be sealed to the casing with a lead or rubber packer ring after the insertion of the gravel pack. In order to rehabilitate a well, it is, of course, necessary to know exactly how far down the casing extends. Although records are usually kept concerning the construction of a particular well, there are occasions when no information is available to show the depth of the well casing. This information is necessary but is now only obtainable by electrically logging the well, an operation that is both expensive and time consuming.